The cell contents of a primary (non-rechargeable) alkaline cell typically contain an anode comprising zinc, alkaline electrolyte, a cathode comprising manganese dioxide, and an ion permeable separator between the anode and cathode. The alkaline electrolyte is typically an aqueous solution of potassium hydroxide, but other alkali solutions of sodium or lithium hydroxide may also be employed. The cell contents are typically housed in a cylindrical steel container. In the cathodes of conventional Zn/MnO.sub.2 alkaline cells the manganese dioxide composition is typically between about 70 and 87 percent by weight. Graphite and aqueous KOH solution (7-11 Normal) can be added to the manganese dioxide to form a cathode mixture. Such mixtures form a moist solid mix which can be fully compacted into the cell casing using plungers or other such compacting devices forming a compacted solid cathode mass in contact with the cell casing. The cathode material can be preformed into the shape of disks which are inserted into the cell in stacked arrangement, for example, as shown in U.S. Pat. No. 5,283,139, and then recompacted.
The anode material can comprise zinc particles admixed with zinc oxide and conventional gelling agents, such as carboxymethylcellulose or acrylic acid copolymers, and electrolyte solution. The gelling agent holds the zinc particles in place and in contact with each other. The ion permeable separator material, typically of cellulosic material or combination of polyvinylalcohol and cellulosic fibers, can be placed over the inside surface of the cathode before insertion of the anode material. A conductive metal nail, known as the anode current collector, is typically inserted into the anode material and is in electrical contact with an end plate which forms the cell's negative terminal.
There is a growing need to make primary alkaline cells better suitable for high power application. Modern electronic devices such as cellular phones, digital cameras and toys, flash units, remote control toys, camcorders and high intensity lamps are examples of such high power applications. Such devices require high current drain rates of between about 0.5 and 2 Amp, typically between about 0.5 and 1.5 Amp. Correspondingly, they require operation at power demands between about 0.5 and 2 Watt.
Manganese dioxide is commonly employed as a cathode active material in commercial batteries including heavy duty cells and alkaline cells, such as zinc/Mno.sub.2 alkaline cells comprising an aqueous alkaline electrolyte or lithium/Mno.sub.2 cells comprising an organic nonaqueous electrolyte. Conventional alkaline cells have solid cathodes comprising battery grade particulate manganese dioxide. Battery grade manganese dioxide as used herein refers to manganese dioxide generally having a purity of at least about 91 percent by weight. Electrolytic MnO.sub.2 (EMD) is the preferred form of manganese dioxide for alkaline cells because of its high density and since it is conveniently obtained at high purity by electrolytic methods. EMD is typically manufactured from direct electrolysis of a bath of manganese sulfate and sulfuric acid. Processes for the manufacture of EMD and its properties appear in Batteries, edited by Karl V. Kordesch, Marcel Dekker, Inc., New York, Vol. 1, (1974), p. 433-488. Battery grade manganese dioxide known as chemical manganese dioxide (CMD), a chemically synthesized manganese dioxide, has also been used as cathode active material in electrochemical cells including alkaline cells and heavy duty cells.
However, manganese dioxide is actually a non stoichiometric material more accurately written as Mn.sup.+4.sub.1-x-y Mn.sup.+3.sub.y V.sub.x O.sub.2-4x-y (OH).sub.4x+y, where V stands for vacancy on the cationic site and the OH group indicates the hydroxyl defects present in MnO.sub.2. The non-stoichiometry is due to the presence of hydroxyl groups which results in Mn.sup.+3 defects for the sake of charge neutrality. Because of the Mn.sup.+3 defects and presence of hydroxyl groups associated therewith, the formula for conventional battery grade manganese dioxide, whether in the from of EMD or CMD, is more accurately represented by the overall formula MnO.sub.x, 1.92&lt;x&lt;1.96. (The formula MnO.sub.x as used herein is understood to be an overall representation of the above complex formula.) Thus, if the overall formula is MnO.sub.1.92 the average valence of manganese is +3.84 (assuming a valence of -2 for oxygen) and if the formula is MnO.sub.1.96 the average valence of manganese is +3.92. (The term average valence as used herein is intended to be a simple arithmetic average, that is, the sum of the valence of each manganese atom in the manganese dioxide sample divided by the total number of manganese atoms.) Some forms of CMD can be synthesized to have lower average levels of oxidation, for example MnO.sub.x, wherein x can be &gt;1.5. More generally manganese dioxide can be synthesized having an overall formula MnO.sub.x, wherein 1.5&lt;x&lt;2.0.
U.S. Pat. No. 2,956,860 (Welsh) discloses a chemical process for the manufacture of battery grade MnO.sub.2 by employing the reaction mixture of MnSO.sub.4 and an alkali metal chlorate, preferably NaClO.sub.3. This process is known in the art as the "Sedema process" for manufacture of chemical manganese dioxide (CMD).
It would be desirable to treat conventional battery grade manganese dioxide (EMD or CMD) used as active material in electrochemical cells, particularly alkaline cells to remove the MnOOH defects in the EMD or CMD structure and increase the average valence of manganese to approach more closely absolute +4. This would theoretically improve the specific capacity (milliAmp-Hr/g) of the manganese dioxide in the cell and could make the treated manganese dioxide more suitable for high power application.